Sending mail from Word or Excel getting error

The system we currently have in place is Outlook 2003. We run this on Windows
XP SP 2. The issue we are having is we use to have two email accounts for
every user. An internal and an external email. We have gone away from the
internal and now only use the external. The problem we are having is when you
try to send an email from either word or excel it defaults to the old
address. This is bad cause when the person who receives the email if they
reply to it the orginal sender never gets the reply.

Is there a way to correct this?
If there is, is it a per user basis?

Thanks in advance